1. Field of the Invention
The present invention is generally directed to lithography, and more particularly to testing lithographic apparatuses.
2. Background Art
A null corrector is a device that may be used to analyze an optical surface, such as a spherical optical surface or an aspherical optical surface. A typical null corrector includes an interferometer, reference optics (such as one or more mirrors and/or one or more lens), and a test optical surface. The interferometer provides a test beam and a reference beam. The test beam is conditioned by the reference optics and then incident on the test optical surface. If the test optical surface is “ideal” (i.e., fabricated within predetermined specifications), the beam conditioned by the reference optics impinges on the test optical surface at (approximately) normal incidence. A wavefront that impinges on a test optical surface at (approximately) normal incidence is referred to herein as a “null wavefront.” The test optical surface further conditions the test beam. Thereafter, the test beam reenters the interferometer and interferes with the reference beam resulting in a fringe pattern, or more specifically an interferogram. Based on the interferogram, the test optical surface may be analyzed to determine if it has been fabricated within the predetermined specifications.
Null correctors may be built using conventional optical elements or diffractive optical elements. Conventional optical elements include refractive elements, reflective elements, and/or a combination of refractive and reflective elements. Null correctors that include refractive and/or reflective optical elements have limitations with respect to accuracy and the types of aspheric surfaces that can be tested. Diffractive null correctors can typically test many different types of aspherical optical surfaces and are typically more accurate than null correctors that include conventional optical elements. A diffractive null corrector includes a diffraction grating that is configured to produce a null wavefront corresponding to an optical surface being tested.
A problem with null correctors is that they are expensive to fabricate. A typical null corrector is designed to analyze a specific aspheric profile. It is rare that a null corrector can be used to test an optical surface other then the one it was originally designed to analyze. The non-recurring cost associated with a null corrector significantly adds to the cost and delivery schedule of an optical element that is analyzed by a null corrector. Because aspherical optical surfaces are typically analyzed using a null corrector, typical asphere-based optical designs cost more and take longer to fabricate than all-sphere optical designs. Thus, although an asphere-based optical design typically has superior optical quality compared to an all-sphere optical design, cost and schedule usually drive customers to select all-sphere optical designs.
To avoid the typical high cost and schedule delays associated with asphere-based optical designs, software-based null correctors have been used to test the aspherical optical elements. In such software-based null correctors, the effect of the aspheric surface is not canceled out prior to entering the interferometer. Rather, an aberrated wavefront is allowed to propagate through the interferometer. The amount of error a perfectly fabricated aspheric surface would cause in the interferometer is calculated using a ray tracing program and then subtracted from the wavefront that is actually measured. The difference between the actual and predicted wavefronts is used as a measure of the surface error in the aspherical optical surface being analyzed. However, this technique only works over a limited range of asphericity because aberrations due to the asphericity can cause rays propagating through the interferometer system to be vignetted, and thereby prevent the whole optical surface from being analyzed.
Given the foregoing, what is needed is an improved diffractive null corrector that may be re-used to test optical surfaces of various shapes. Such an improved diffractive null corrector should allow for a reduction in the cost of fabricating diffractive null correctors. Furthermore, such an improved diffractive null corrector should allow for compensation of unexpected errors or problems occurring in the optics of the diffractive null corrector.